1. Field of the Disclosure
The present disclosure relates generally to an image fixing assembly of an image forming apparatus, and more specifically, to controlling overheating in an image fixing assembly of an image forming apparatus in order to enhance throughput of the image forming apparatus while printing media sheets.
2. Description of the Related Art
In an image forming apparatus, such as an electrophotographic printing apparatus, unfused toner images (i.e., latent images) are fixed on a media sheet by an image fixing assembly of the image forming apparatus. Typically, an image fixing assembly of an image forming apparatus includes a heating unit having a heating element and a fusing member, and a backup member abuttingly coupled to the fusing member of the heating unit. Further, the fusing member of the heating unit may be in the form of either a fuser roll or a fuser belt. Furthermore, the heating element of the heating unit may be in the form of either a lamp or a ceramic heater. An image fixing assembly having a fuser roll enclosing a lamp may be referred to as “hot roll fuser system” and an image fixing assembly having a fuser belt enclosing a ceramic heater may be referred to as “belt fuser system”.
As described above, the backup member of the image fixing assembly is abuttingly coupled to the fusing member for configuring a nip portion therebetween. The media sheet carrying the unfused toner images thereon passes through the nip portion in order to allow fixing of the unfused toner images. Specifically, when the media sheet carrying the unfused toner images passes through the nip portion, the heating element provides heat to the media sheet and the backup member applies pressure onto the media sheet to enable fixing of the unfused toner images onto the media sheet.
In an instance when a narrow media sheet, such as an envelope, passes through the nip portion, the narrow media sheet does not extend across the full width of the fusing member and the backup member. Accordingly, thermal energy accumulates at portions of the fuser member and the backup member that are not in contact with the narrow media sheet. Specifically, the portions of the fusing member and the backup member that are not covered by the narrow media sheet tend to accumulate more heat as opposed to portions of the fusing member and the backup member that are covered by the narrow media sheet. As a result, a thermal gradient is generated on the fusing member and the backup member of the image fixing assembly. Further, there is a gradual increase in the thermal gradient in such an image fixing assembly after printing several consecutive narrow media sheets. Accordingly, high temperatures at the portions of the fusing member and the backup member where a narrow media sheet is not present may cause damage to the image fixing assembly and components thereof.
In addition, a hot roll fuser system employed as the image fixing assembly in an image forming apparatus is associated with a high thermal mass. Specifically, the hot roll fuser system includes a fuser roll as the fusing member and a backup roll as the backup member, and both the fuser roll and the backup roll are, in general, manufactured from thick metal cores that are surrounded by rubber layers. Accordingly, the hot roll fuser system is associated with a large thermal mass due to the use of the thick metal cores that are surrounded by the rubber coating for manufacturing the fuser roll and the backup roll. A thermal gradient generated in such a hot roll fuser system is related to the thermal mass of the hot roll fuser system, and the respective thicknesses of the metal cores and the rubber coating of the fuser roll and the backup roll. Further, in a typical hot roll fuser system, the thermal gradient is generated slowly after printing several consecutive narrow media sheets. However, fixing of a first image during printing of media sheets using the hot roll fuser system, which employs the fuser roll having the large thermal mass, becomes time-consuming, as there may exist a delay in raising the temperature of the fuser roll prior to printing. Specifically, the large thermal mass of the hot roll fuser system leads to a long warm-up time for printing a first media sheet.
Further, printing narrow media sheets may gradually lead to failure of the hot roll fuser system. In such an instance, an inter-page gap may be increased to allow excess heat to dissipate from the fuser roll and the backup roll, and to allow the excess heat to conduct to portions having a lower temperature, particularly, the portions of the fuser roll and the backup roll covered by the narrow media sheets. Typically, the inter-page gap is increased after a first count of narrow media sheets, and may again be increased one or more times after subsequent counts of narrow media sheets. As a result, throughput associated with the printing of the narrow media sheets is reduced as opposed to throughput associated with printing of full width media sheets. The term, “inter-page gap,” relates to the separation between successive media sheets.
Alternatively, a belt fuser system, which employs a fuser belt as the fusing member, is associated with a thermal mass lower than that of the hot roll fuser system. Specifically, the belt fuser system employs an amount of metal for manufacturing the fuser belt that is lower than the amount of metal required for manufacturing the fuser roll of the hot roll fuser system. Accordingly, the belt fuser system is associated with a lower thermal mass. Further, the lower amount of metal in the belt fuser system results in a lower axial thermal conductivity as opposed to the hot roll fuser system. Furthermore, the lower thermal mass leads to a short warm-up time for printing a first media sheet as opposed to the printing of the first media sheet using the hot roll fuser system. However, the lower axial thermal conductivity of the fusing member of the belt fuser system poses difficulty while printing narrow media sheets. Specifically, a high thermal gradient is generated after successive printing of narrow media sheets due to the lower axial thermal conductivity, which may lead to a failure of the belt fuser system. Accordingly, the inter-page gap may be increased in the belt fuser system to allow excess heat to dissipate from the fusing member and the backup member, and to allow the excess heat to conduct to portions having lower temperature, particularly, the portions of the fusing member and the backup member covered by the narrow media sheets. Consequently, generation of a high thermal gradient may severely impact throughput of the belt fuser system. Specifically, throughput for printing the narrow media sheets may be reduced by a factor of 10 as opposed to throughput for printing full width media sheets. More specifically, by increasing the inter-page gap, throughput associated with the belt fuser system is reduced in order to avoid damage to the belt fuser system by overheating of various components thereof.
Moreover, a delay before printing full width media sheets may be required after printing several narrow media sheets using either the hot roll fuser system or the belt fuser system. Additionally, generation of the thermal gradient, particularly, generation of a high temperature on portions of the fusing member and the backup member may cause a defect in print quality, as unfused toner tends to stick to the heating unit instead of properly adhering to a media sheet. This problem may be prominent in the belt fuser system, since the belt fuser system requires a longer time period for recovering from a state with a high thermal gradient, due to less conduction of heat between the portions not covered by media sheets and the portions covered by the media sheets.
Various techniques have been developed in order to reduce a thermal gradient generated in an image fixing assembly for controlling overheating in the image fixing assembly. One such conventional technique to reduce the thermal gradient generated on a fusing member enclosing a heating element, and a backup member of an image fixing assembly includes turning off the heating element when a narrow media sheet exits the nip portion between the fusing member and the backup member. Specifically, the heating element is turned off to allow the fusing member and the backup member to dissipate heat from portions thereof that are not in contact with the media sheet, thereby reducing the thermal gradient generated in the image fixing assembly. Accordingly, printing of full width media sheets after printing of narrow media sheets using such a technique proves to be time-consuming due to the delay required for turning off of the heating element for reducing the thermal gradient and then turning the heating element on for maintaining a requisite temperature prior to subsequent rounds of printing full width media sheets after printing narrow media sheets.
Another conventional technique to control overheating in an image fixing assembly employs a use of a temperature sensing member, such as a thermistor. The temperature sensing member may be operatively coupled to one of a fusing member and a backup member of the image fixing assembly to detect a thermal gradient generated thereon. Specifically, the temperature sensing member may be coupled to the backup member for sensing the temperature of the backup member. Further, the temperature sensing member may be coupled to a controller, which is further coupled to a heating element of a heating unit of the image fixing assembly. The controller controls the operation of the heating element based on the temperature of the backup member. Further, the controller maintains the heating element at or near a target temperature when the temperature of the backup member is within a predefined temperature range. For example, when the temperature sensing member detects a high temperature on a portion of the backup member not covered by a narrow media sheet, the controller may either modify (i.e., reduce) the target temperature of the heating element or may deactivate the heating element. Alternatively, the controller may control the operation of the heating element based on the temperature of the backup member during fusing of at least one initial narrow media sheet and during fusing of at least one subsequent narrow media sheet. Accordingly, inter-page gap may be increased in order to control the thermal gradient for controlling overheating in the image fixing assembly. Additionally, when the thermal gradient is reduced to a predetermined value, a full width media sheet may be printed by the image fixing assembly.
Alternatively, in absence of the temperature sensing member, the inter-page gap may be increased after printing of a pre-determined number of narrow media sheets. Further, in the absence of the temperature sensing member, a pre-determined time delay may be introduced before continuing printing of narrow media sheets. Accordingly, an increase in the inter-page gap and/or introduction of the pre-determined time delay results in reduction of throughput for printing narrow media sheets and full width media sheets by the image fixing assembly.
Accordingly, there is a need for controlling overheating in an image fixing assembly of an image forming apparatus in order to enhance throughput of the image forming apparatus while printing narrow media sheets and full width media sheets.